These actions are performed in order, but no triggered abilities resolve in between them and no player may cast spells or activate abilities in between them. This means that if a permanent's ability triggers on a player losing life, that permanent will be exiled first and won't be on the battlefield when your life total becomes 1. Leaves-the-battlefield triggers won't resolve until you're done resolving Worldfire entirely.

7/1/2012

If your life total is greater than 1 when Worldfire resolves, the change in life total will count as life loss. Other effects that interact with life loss will interact with this effect accordingly.

7/1/2012

Worldfire won't be exiled. It will go to your graveyard as the final part of its resolution.